I am always very sceptical of QR promotional codes (to put it lightly which is merited given the OP's post above).

I did a like-for-like comparison for LHR-SIN (via DOH) from 12 January returning 27th February and with the preferred flights there was a 9.1% discount using SAVENOW (£4569 vs £5029).

Not the 25% I was hoping for. Yes I know it says "up to 25%".